The role of the hydrodynamic regime in the distribution of

Limnetica, 29 (2): x-xx (2011)
Limnetica, 33 (1): 1-12 (2014)
c Asociación Ibérica de Limnología, Madrid. Spain. ISSN: 0213-8409
The role of the hydrodynamic regime in the distribution of the invasive
shrub Baccharis halimifolia (Compositae) in Oyambre Estuary
(Cantabria, Spain)
Diego Frau1,∗ , Bárbara Ondiviela Eizaguirre2 , Cristina Galván Arbeiza2
and José Antonio Juanes de la Peña2
1
Plankton Department. Limnology National Institute (CONICET-UNL), Ciudad Universitaria, 3000 Santa Fe,
Argentina.
2
Environmental Hydraulics Institute “IH Cantabria”, Universidad de Cantabria. C/ Isabel Torres no 15. Parque
Científico y Tecnológico de Cantabria. 39011, Santander, Spain
∗
2
Corresponding author: diegofrau@gmail.com
Received: 15/05/2013
Accepted: 10/10/2013
ABSTRACT
The role of the hydrodynamic regime in the distribution of the invasive shrub Baccharis halimifolia (Compositae) in
Oyambre Estuary (Cantabria, Spain)
Coastal estuaries are affected by an increasing number of human interventions, including the introduction of invasive species.
Baccharis halimifolia is endemic to the east coast of North America and in recent decades has colonised a large number
of estuaries in northern Spain. Although hydrodynamic conditions play an important role in the creation of niches for plant
species in salt marshes, their interaction with the spatial distribution of B. halimifolia remains poorly studied. This study
identified the main hydrodynamic variables controlling the distribution of this species in Oyambre estuary (Cantabria, Spain)
and used these variables to model the habitat of the plant. B. halimifolia develops in areas that are inundated < 26 % of the year,
with salinity < 25 g/L, and water speed and water flow < 0.1 m/s and < 0.85 m3 /s, respectively. Habitat suitability modelling
showed that this plant is not equally represented throughout the estuary; preferring areas where hydrodynamic values are lower
than average. The plant also showed medium tolerance to the present hydrodynamic conditions (tolerance ≪ 1), preferring
areas that are inundated 0 to 21 % of the year, with salinity between 0 and 26 g/L. Changes in hydromorphologic conditions
may have a predictable impact on the type and distribution of vegetation in salt marshes. B. halimifolia will remain in areas
where tidal influence is reduced, competing with native vegetation.
Key words: Baccharis halimifolia, habitat suitability, hydrodynamic conditions, invasive species, salt marshes.
RESUMEN
El rol del régimen hidrodinámico en la distribución del arbusto invasor Baccharis halimifolia (Compositae) en el estuario
de Oyambre (Cantabria, España)
Un número creciente de intervenciones humanas afectan los estuarios costeros, siendo la introducción de especies invasoras
una de las más notables. Baccharis halimifolia es una planta nativa de la región costera este de Norte América, que en las
últimas décadas ha colonizado un gran número de estuarios en el norte de España. Si bien las condiciones hidrodinámicas
juegan un rol preponderante en la creación de nichos ecológicos para la vegetación de marisma, la manera en que éstas
influyen en la distribución de B. halimifolia en las marismas colonizadas, ha sido poco estudiado. Este trabajo tuvo por
objetivos identificar las variables hidrodinámicas más importantes que controlan la distribución de esta especie en el estuario
de Oyambre (Cantabria, España), para luego usar esta información en la realización de un modelo de hábitat potencial. B.
halimifolia se desarrolla en áreas con tiempo inundado (% al año) que no superan el 26 %, salinidades menores a 25 g/l
y velocidad del agua y caudal circulante menores a 0.1 m/s y 0.85 m3 /s respectivamente. El modelo de hábitat potencial
mostró que esta planta no se encuentra representada de forma homogénea en el dominio del estuario, distribuyéndose
de forma preferencial en áreas donde los valores medios de las variables hidrodinámicas consideradas son menores a las
condiciones hidrodinámicas medias en todo el estuario. La planta también mostró una moderada tolerancia a las condiciones
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Frau et al.
hidrodinámicas (Tolerancia ≪ 1) teniendo como áreas altamente potenciales de ser habitadas, aquellas con tiempo inundado
(% al año) entre 0 y 21 % y salinidad entre 0-26 g/l. Los cambios en las condiciones hidrodinámicas pueden tener un impacto
predecible en el tipo de vegetación y su distribución en marismas. Los resultados sugieren que B. halimifolia se mantendrá
en aquellas áreas con reducida influencia mareal, compitiendo con la vegetación nativa.
Palabras clave: Baccharis halimifolia, habitat potencial, condiciones hidrodinámicas, especies invasoras, marismas.
INTRODUCTION
Coastal estuaries are highly productive systems
that combine beaches, dunes and salt marshes
(Adam, 2002). Nowadays, several estuaries support a growing human population and a growing
number of industries, buildings and tourism facilities. As a consequence, the vegetation and natural structure of the estuaries are receding (Canteras et al., 2003; Heywood & Iriondo, 2003).
Such anthropic perturbations facilitate the invasion of exotic plants, constituting an extremely
difficult problem (Coblentz, 1990). As they are
commonly highly opportunistic pioneers, exotic
plants can produce large numbers of seeds and
can take advantage of the absence of natural enemies. Having a high rate of development, they
can colonise modified environments and compete
against native plants, changing the appearance,
structure and function of the natural plant community. According to GEIB (2006), Baccharis
halimifolia (Linneo, 1737) is one of the 20 most
dangerous invasive animal and plant exotic invasive species in Spain.
B. halimifolia grows near dunes, fresh and
salt-water bodies and deserted areas of Massachusetts, Florida, Arkansas and Texas (USA;
Van Deelen, 1991). It was introduced to France
in 1863 and to Australia in the 1890s (Caño et
al., 2013). It was reported in the Iberian Peninsula in 1941, and was recorded for the first time
in Cantabria around 1953. Since then, Oyambre,
Noja, and Asón estuaries have been colonised by
this species (Campos et al., 2004).
Several studies have analysed the ecology of
this plant. Westman et al. (1975) were the first
authors to study seed growth under different environmental conditions. Ewe & Sternberg (2002)
studied the seasonal water use of this plant, show-
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ing that B. halimifolia shifted from deep groundwater to shallow soil water in the wet season. In
addition, Young et al. (1994) found an average
tolerance to brackish soils (between 2 to 5 g/L),
being flood level and salt spray other relevant factors for to the distribution of this plant (Tolliver
et al., 1997; Griffits, 2006; Zinnert et al., 2012).
Environmental factors like water speed, flow and
salinity, which are relevant to the distribution of
native plant marshes (Crain et al., 2004), might
influence the distribution of B. halimifolia, but to
the best of our knowledge, this effect has not yet
been studied for this invasive species.
Exotic species present a threat to the biodiversity, conservation and restoration of many natural
areas and often drive ecological changes that may
be irreversible. Understanding trends in the spread
and density of invasive exotic species, including
the impact of control and management activities, is necessary to manage invasive species and
should be a vital part of large-scale ecosystemrestoration programs (Doren et al., 2008).
Therefore, the aims of this work were to analyse the influence of the hydrodynamic variables
on the distribution of the invasive shrub B. halimifolia in a recently restored estuary and to define
the habitat of B. halimifolia based on hydrodynamic variables in Oyambre estuary.
MATERIALS AND METHODS
Study area
The Oyambre estuary is located in the West of
Cantabria (northern Spain) and has two branches,
Rabia and Capitán. It is a natural park, is included
in the SCI (Site of Community Importance;
“Rías Occidentales y Dunas de Oyambre”), has a
Influence of the hydrodynamic regime on the distribution of Baccharis halimifolia
Figure 1. Geographic localisation of Oyambre estuary. Both
branches of the estuary (Capitán and Rabia) and the two restoration activities carried out in 2009 are indicated: un-modified
roads by continuous lines and open roads by dashed lines.
The coverage of B. halimifolia is indicated in the legend. Localización geográfica del estuario de Oyambre. Ambos brazos del estuario (Capitán y Rabia), así como las medidas de
restauración realizadas en el año 2009 se encuentran indicadas: caminos sin modificar representados con líneas continuas, caminos modificados con línea discontinua. El área de
cobertura de B. halimifolia es indicado en la leyenda.
relative surface of 100 ha, and a perimeter of
13.6 km. Until the first half of 2009, the estuary
included four dikes to control the tidal flux, and
B. halimifolia was distributed in approximately
35 % of the estuary area, with > 80 % coverage
in those areas. In July 2009, the plant was mechanically removed, and in October of the same
year, structural modifications were performed on
a road that also functioned as a dike for the two
estuary branches. This new design allows the natural tidal flux to penetrate the estuary despite the
presence of the road (Fig. 1).
Moreover, estuaries on this coast are characterised by large intertidal surfaces and dominated
by the tidal dynamic, making them well-mixed.
Oyambre is a vertically homogeneous, fluvial fitted valley. It is exposed to a semidiurnal tide,
with a mesotidal range varying from 0.5 m during
neap tides to 2.3 m during spring tides.
Vegetation data
A distribution map of B. halimifolia in 2011 was
developed at a 1:5000 scale using a combination
of three aerial photographs. The resulting picture
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3
Figure 2. Bathymetric plot of Oyambre estuary. Negative
numbers indicate altitudes higher than sea level, with areas
marked −100 lying outside the estuaries. Control points, at
which numerical modelling data were obtained, are indicated
by stars (every single star represents a 25 × 25 m cell). Mapa
batimétrico del estuario de Oyambre. Los números negativos
indican altitudes por encima del nivel del mar, −100 representa
áreas por fuera del estuario. Los puntos de control donde se
obtuvo información hidrodinámica se encuentran representados con estrellas (cada estrella representa una celda de 25 × 25
metros).
was divided into polygons, which were labelled
according to a combination of colour and texture to identify the main species of vegetation. To
verify these identifications and to determine percent coverage of B. halimifolia and other natural
salt marsh vegetation, on-the-ground verification
was conducted during two later field studies in
November. These data were then included in the
image data. All versions were created using ArcGis
software 9.3 (Cantabria University License, 2011).
Environmental data
According to the literature (Tolliver et al., 1997;
Griffits, 2006; Zinnert et al., 2012), flooding frequency and salinity are the two major variables
that control the distribution of B. halimifolia. In
Oyambre estuary the hydrodynamic regime was
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Frau et al.
altered until 2009, yet the natural regime has
now been partially restored. Thus, 5 hydrodynamic variables that might affect the development and distribution of B. halimifolia were calculated: percent of the year inundated, water flow
and water speed, salinity, and freshwater run-off.
Numeric flood modelling was performed with
an H2D model, and salinity was modelled with an
AD2D model (Bárcena et al., 2011). Both models were performed with the same dike aperture
(90 m for Capitán and 100 m for Rabia) using
a bathymetric grid of 25 × 25 m and a cell extension of 165 × 141. The hydrodynamic modelling was performed to simulate 1 year (8760
hours) with a variable-amplitude wave (2.3 m
maximum altitude); the saline model was performed at 25 ◦ C and an initial salinity of 35 g/L.
Only the astronomic wave was considered in the
simulation.
With velocity, water flow and height of the
free surface with respect to the average sea level
(ETA) obtained during simulation, the average
velocity, average water flow, and average time
inundated (time flooded/total time of simulation*100) were estimated at 31 points in the estuary (Fig. 2). Salinity modelling was performed
using H2D hydrodynamic data to obtain an
average concentration of salt (g/L) at every point.
Finally, freshwater run-off was estimated
using the rational method (Chow et al., 1994).
Rainfall intensity was calculated with the Temez
(2002) formula, and the average maximum rainfall intensity was obtained using the MAXIM
software (De Salas et al., 2005). The water
run-off coefficient, necessary for the calculation
was taken from the nearest basin (Escudo Basin
River, GESHA, 2005). The rational method does
not consider the interaction of run-off with different types of soils, evaporation processes and
vegetation use, which could affect the volume of
water that runs over the basin.
To evaluate the threshold distributions of
the plant, percent coverage and hydrodynamic
variables in the 31 control points were plotted.
With the 5 hydrodynamic variables obtained at
the 31 control points, 5 eco-geographical variables (EGVs) in raster format were introduced
into the habitat suitability model. Those eco-
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geographical variables were the result of the interpolation of data for every point in the estuary (the 31 control points), using the inverse
distance-weighted method (IDW) in ArcGis 9.3.
IDW interpolation is based on the assumption
that objects that are close to one another are
more alike than those that are farther apart. To
predict a value for any unmeasured location,
IDW uses the measured values from surrounding
locations. Values from closer locations have
more influence on the predicted value than those
from locations farther away. Thus, IDW assumes
that each measured point has a local influence that diminishes with distance.
Habitat-suitability model
The ecological niche factor analysis (ENFA) developed by Hirzel et al. (2002) computes suitability functions by examining the species distribution across eco-geographical variable space. This
approach determines the relationships between
variables and establishes combinations of these
variables to produce uncorrelated factors (Valle
et al., 2011). The first factor is defined as the
‘marginality’ (M) of the species niche, which is
the absolute difference between the global mean
(the mean of the variable throughout the study
area; Bryan & Metaxas, 2007) and the species
mean (the mean of a variable when the species is
present) for each environmental variable (Hirzel
et al., 2002) and is calculated as
mi =
|mGi − mSi |
1.96σGi
where mi is the marginality of a particular environmental variable, mGi is the global mean of the
variable, mSi is the mean of the variable in the
species range, and σGi is the standard deviation
of the global distribution of the variable (Bryan
& Metaxas, 2007).
Combining the mi of individual environmental variables, ENFA computes an overall global
M; Hirzel et al., 2002; Reutter et al., 2003). M
ranges between 0 and 1, with larger values indicating that the species is not equally distributed
throughout the environment.
Influence of the hydrodynamic regime on the distribution of Baccharis halimifolia
Specialisation (S) is the second factor and
indicates how restricted the species niche is in
relation to the study area (‘global’ area). It is
defined as the ratio of variance in the global
distribution to variance in the species distribution
of the environmental variable (Hirzel et al., 2002;
Reutter et al., 2003), and it is calculated as
λi = σGi /σSi
where λi is the S for a particular environmental variable and σGi and σSi are the species and
global standard deviations of the variable, respectively. As for M, S for individual environmental variables is combined to compute an overall
global S. S ranges from 1 to N, with the niche becoming narrower as S increases. Tolerance (T),
which is the inverse of S, ranges from 0 to 1 and
represents the general tolerance of the species to
environmental conditions. Values near 1 indicate
a higher tolerance to natural conditions.
5
Marginality, specialisation and tolerance for
the whole estuary were obtained as results of the
ENFA model. The total variance of the B. halimifolia distribution was derived from the combination of all eco-geographical variables (EGVs).
For M, EGVs are sorted by decreasing the absolute value of coefficients. Positive values indicate
that the species prefers locations with higher
values on the corresponding EGV than average
in the study area, and vice versa. Coeficient signs
have no relevance on the specialisation factors
(e.g., specialisation 1, 2, 3). The sum of the factors (marginality and specialisation) explains 100 %
of the total variance in the distribution of the plant.
For the habitat suitability model, B. halimifolia distribution data and environmental data
(mean water speed, mean water flow, percent of
the year inundated, salinity and water run-off)
from 2011, were fed into BioMapper 4.0 software (www.unil.ch/biomapper/) as a raster-based
grid file (eco-geographical variables), with a
Figure 3. (a) B. halimifolia coverage in November 2011, Oyambre estuary domains. The coverage values, as adjusted after field
verification, are indicated on the left-hand side of the figure. (b) Habitat suitability map for 2011. Accuracy levels are indicated for
the whole estuary. (a) % de cobertura de B. halimifolia en Noviembre 2011 dentro de los dominios del estuario de Oyambre. Los diferentes valores de % de cobertura, ajustados en campo se encuentran indicados en la porción izquierda de la figura. (b) Mapa de hábitat potencial para el año 2011. Los niveles de potencialidad de hábitat se encuentran calculados para toda la superficie del estuario.
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Frau et al.
horizontal resolution of 25 × 25 m and the same
extent. Subsequently, a box-cox algorithm was
applied to normalise all eco-geographical variables. The habitat suitability map was produced
using a global M value (0 to 100) for all environmental factors in each cell that indicates the most
probable distribution of the plant according to the
real distribution and eco-geographical variables
in the model.
The habitat suitability map was rescaled using
the isopleth method and the validation of the
model was done by the Boyce Index (Boyce et
al., 2002), which indicates a good adjustment
of the prediction when the index value is > 0.7.
Finally, the ENFA results were plotted using the
average algorithm. This algorithm assumes that
the mean value for the environmental variable
within the species distribution is approximately
the same as the value in the study area (Hirzel et
al., 2002; Hirzel & Arletta, 2003).
between branches were statistically significant
(Mann-Whitney test, Z = −2.052, ρ = 0.040).
Analyses of salt-marsh vegetation showed that
the principal area of distribution of B. halimifolia
in Rabia was dominated by Juncus maritimus
(Lamarck, 1789) (average 78 %). In Capitán,
those areas were dominated by Halimione portulacoides (Aellen, 1938) (average 65 %).
B. halimifolia presented a threshold distribution pattern, showing greater coverage in areas
that are inundated < 20 % of the year. The plant
did not invade areas that are inundated > 26 %
of the year (Fig. 4a), and was distributed in
areas where water speed and water flow were
< 0.1 m/s and < 0.85 m3 /s, respectively (Fig. 4b
and Fig. 4c). Furthermore, B. halimifolia grew
in areas where the median salinity was < 25 g/L
(Fig. 4d), but did not show a specific threshold
distribution in terms of run-off (Fig. 4e).
Habitat suitability model
RESULTS
Plant pattern distribution
In 2011, B. halimifolia was present in 31 % of
the total area of the estuary, representing 33.49 %
of the Rabia surface and 46.66 % of the Capitán
surface. Where the species was present, coverage
was between 1 and 90 %. In Capitán, where the
flood restoration was less important, the plant
had coverage between 10 and 80 %. In contrast,
in Rabia, where flood restoration was more
significant (see Fig. 1), coverage was between
1 and 15 % (Fig. 3a). The coverage differences
Marginality (M) was 0.57, global specialisation (S)
was 2.22, and global tolerance (T) was 0.45. M
indicates that the estuary presents a combination of
average hydrodynamic conditions that differ from
the average hydrodynamic conditions preferred
by the plant. S ranged between 0 and N and in
combination with T (1/S) indicates that the plant
is restricted in its distribution area. With 4 ecogeographical factors, the ENFA model explained
96.4 % of the total variability, with the first factor
(marginality) covering 54.4 % of the explained variability. Factor 2 (specialisation 1) explained
18.6 %, Factor 3 (specialisation 2) 10.6 % and
Factor 4 (Specialisation 3) 7.8 % (Table 1).
Table 1. Variance explained by the first four ecological factors (out of 5) and coefficient values for the 5 eco-geographical variables
(EGVs) used. Key: Wat. fl.= water flow; Wat. sp. = water speed; T. in. % = percent of the year inundated; Saln. = salinity. Varianza
explicada por los primeros cuatro factores ecológicos (de un total de 5) con los coeficientes obtenidos para cada una de las variables
eco-geográficas (EGVs). Abreviaturas: Wat. fl. = caudal; Wat. sp. = velocidad; T. in. % = tiempo inundado (% a al año); Saln. =
salinidad.
Marginality (59.4 %)
Spec. 1 (18.6 %)
Spec. 2 (10.6 %)
Spec. 3 (7.8 %)
Wat. fl. (−0.529)
Run-off (0.524)
Wat. sp. (−0.422)
T. in. % (−0.378)
Saln. (−0.353)
T. in. % (0.807)
Saln. (−0.545)
Run-off (0.157)
Wat. fl. (−0.135)
Wat. sp. (0.098)
Wat. sp. (0.834)
T. in. % (−0.425)
Wat. fl. (−0.255)
Run-off (0.199)
Saln. (0.136)
Wat. fl. (−0.74)
Wat. sp. (0.586)
Run-off (−0.315)
Saln. (−0.091)
T. in. % (0.03)
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Influence of the hydrodynamic regime on the distribution of Baccharis halimifolia
The most important (highest M values) variables controlling the presence of B. halimifolia
were water flow (−0.53), run-off (0.52) and water
speed (−0.42). The model showed that this plant
prefers areas where the average water flow and
water speed are lower than the values obtained
for the whole estuary and areas where the water run-off is higher than in the estuary overall. B.
halimifolia also showed specialisation in terms of
percent of the year inundated (0.80) and salinity
7
(0.54) in the Specialisation factor 1 (which explained 18.6 % of the total variance). In Specialisation 2, which explained 10.6 % of the total variance, water speed (0.84) had the greatest value.
In Specialisation 3 (which explained 7.8 % of total variance), water flow (−0.74) was the variable
with the greatest absolute value. These statistics
indicate that percent of the year inundated and
salinity in particular limit the range of B. halimifolia in the estuary. Water speed and water flow
Figure 4. Coverage and distribution of B. halimifolia with respect to time inundated (a), water speed (b), water flow (c), salt
concentration (d) and freshwater run-off (e). Dashed lines indicate the distribution limits of the plant in Oyambre for every parameter.
Valores de porcentaje de cobertura y distribución de B. halimifolia respecto del tiempo inundado (% al año) (a), velocidad del agua
(b), caudal (c), concentración salina (d) y escorrentía superficial (e). La línea de puntos indica el límite de distribución de la planta
en el estuario de Oyambre para cada uno de los parámetros analizados.
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Frau et al.
are secondary control variables according to the
ENFA model.
The results of the cross-validation performed
to evaluate model quality were indicative of its
high predictive capacity (Boyce index: 0.79 ±
0.18), and allowed us to generate an habitat suitability map with ranges between 0 and 100. This
map was divided into 4 intervals of suitability:
from 0 to 25 (unsuitable); from 25 to 50 (scarcely
suitable); from 50 to 75 (moderately suitable);
and from 75 to 100 (highly suitable; Fig. 3b).
These habitat suitability intervals occupy 45 %,
14 %, 7 % and 34 % of the estuary, respectively.
The highly suitable area (from 70 to 100)
represents the most probable distribution of the
plant, indicating a limited range of distribution
of B. halimifolia in terms of percent of the year
inundated (between 0 and 21 %) and water
speed (0-0.08 m/s) but a large range distribution in terms of salinity (0-26 g/L), water flow
(0-2.31 m3 /s) and run-off (0-41 m3 /s).
To simplify the analysis, those four categories
of suitability were grouped into unsuitable areas
(from 0 to 49) and suitable areas (from 50 to
100). Specifically, 41 % of the total area of the
estuary (41 ha) was recognised by the habitat
model as suitable for the growth of B. halimifolia,
and 59 % of the estuary (59 ha) was unsuitable,
lacking the necessary conditions for the growth
of the plant. The results also indicated that those
unsuitable areas corresponded to depths between
0.40 m above sea level and 2.52 m below sea
level, and were associated with flood frequencies
greater than 28 % a year (Fig. 5).
DISCUSSION
Plant pattern distribution
Salt marshes are dynamic, transitional systems
between freshwater and marine environments
that respond to environmental changes (Adam,
2002). The Oyambre estuary has been colonised
by B. halimifolia, an introduced exotic plant,
which was mechanically removed in 2009,
with the modification of the C-6316 road. This
measure allowed greater movement of the tides
in the estuary to penetrate deeper than before
restoration. However, in 2011 the population
Figure 5. Estimated time inundated (% of the year) at different depths in the estuary. Coloured areas represent zones where the
habitat suitability model showed two major groups (Suitable/Unsuitable). Negative values represent points above sea level. Tiempo
total inundado (% al año) estimado para diferentes profundidades del estuario. Las áreas coloreadas indican los dos grandes grupos
indicados por el modelo de hábitat potencial (Adecuado-No adecuado). Valores negativos representan puntos ubicados por encima
del nivel del mar.
15293 Limnetica 33(1), pàgina 8, 15/05/2014
Influence of the hydrodynamic regime on the distribution of Baccharis halimifolia
was recovered, and our results indicate that the
distribution of B. halimifolia is controlled by
flooding, and this plant will remain in those areas
where the influence of flooding is reduced.
In Rabia, coverage was between 1 and 15 %;
and on the other hand, in Capitán where tidal
restoration was less significant (as another
dike still controls the tidal flow) coverage was
between 10 and 50 %. These results suggest a
greater effect of the natural tidal regime of the
estuary in those areas where the seawater penetrates deeper. Espinar (2009) claimed that any
modification to soil salinity, inundation patterns
or drainage processes might alter vegetation
communities in salt marshes. In this particular
case, the restoration of the tidal cycle into the
estuary seems to be an important stressor for
B. halimifolia. When the estuary recovered part
of its natural tidal cycle, the areas that were
dominated by B. halimifolia in 2009, were
re-colonised by native vegetation in 2011. J.
maritimus and H. portulacoides, species that
correspond to the high and middle salt-marsh
zonation, are now dominant species in many areas where B. halimifolia is present. The ability of
plant species to tolerate environmental stress is
especially important in coastal and riverine wetlands, where gradients in hydrology or salinity
determine dominant vegetation patterns (Poulter
et al., 2008). In Oyambre estuary, the increased
tidal influence is changing plant distribution
patterns, allowing the recovery of native salt
marsh vegetation. This finding is consistent with
Cano et al. (2013), who indicated that in other
estuaries of the Cantabrian Sea, B. halimifolia
is distributed in areas where flooding is reduced
and salinity is moderate. Our results are also
consistent with those found on other restored
estuaries (e.g., salt marshes from the Baltic Sea),
where five years after removing the dike line,
nearly 75 % of 350 ha were covered by typical
salt marsh and salt grassland vegetation types
(Bernhardt & Koch, 2003).
Pino et al. (2006) indicated that in a highly
saline habitat, environmental conditions limit the
number, coverage and distribution of alien species,
but in less saline environments with a reduced tidal
influence, alien species are favoured. After dike
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9
modifications in Oyambre estuary, the hydrodynamic conditions changed, and the results
obtained here show that percent of the year inundated, water speed, water flow and salinity control
the distribution and coverage of B. halimifolia.
This plant was distributed in areas that are inundated < 26 % of the year. Tolliver et al. (1997)
demonstrated that exposure to saline water for
longer than 30 consecutive days causes necrosis
in this plant. Our results indicate that B. halimifolia can resist flood exposure for a mean of 94
days of immersion per year, for 6 to 10 hours
a day. Moreover, water speed and flow showed
similar trends indicating that in Oyambre estuary,
B. halimifolia is distributed in areas where the
water speed is < 0.1 m/s and flow is < 0.85 m3 /s.
Water flow and water speed are highly linked,
yet they supply different ecological information
in terms of their effects on plant morphology.
To the best of our knowledge, no studies have
reported the relevance of water flow and water speed to the distribution of B. halimifolia. In
a more general context water movement over a
surface can be associated with soil erosion and
break and deflocculation of clays with effects on
soil stability and thus on vegetation in marshes
(Kozlowski, 1997).
Regarding salinity, Zinnert et al. (2012) established that B. halimifolia can resist high salt
concentrations in its tissues (> 300 mM), maintaining a low photosynthetic activity without
necrosis, and Tolliver et al. (1997) showed that
this plant can resist concentrations of 20 g/L
without mortality. In Oyambre estuary, we found
that B. halmifolia can resist areas where salinity
is below 25 g/L.
Habitat suitability model
Knowledge of the threshold distribution of invasive species can contribute to the understanding
of the factors that control their distributions;
however, prediction of the distribution of alien
plants is difficult, as invasions are unpredictable
and complex (Townsend-Peterson & Vieglais,
2001). One solution was proposed by Ward
(2007), who explained that the study of habitat
can be an important tool in understanding and
10
Frau et al.
managing invasions. In this respect, habitat suitability model (ENFA) showed that B. halimifolia
would not be equally distributed throughout the
estuary, being restricted in its distribution by
the hydrodynamic regime. The estuary does not
present the optimum niche combination (M < 1),
but the plant’s tolerance allows it to persist in
Oyambre (T = 0.45); especially, in elevated areas
(> 0.4 m) where the tidal influence is reduced.
These results are consistent with Caño et al.
(2013) who have indicated that thresholds values
of salinity and waterlogging limit the survival
and spread of B. halimifolia in a halophilous
community.
In addition, ENFA model showed that B. halimifolia tend to grow in those areas where water
speeds is between 0-0.08 m/s (mean, 0.01 m/s)
and water flows is between 0-2.31 m3 /s (mean,
0.18 m3 /s). Conversely, B. halimifolia has moderate tolerance to variations in time inundated
and salinity, preferring areas with low tidal influence (< 28 %), having a considerable tolerance to salinity (0-26 g/L). The ENFA model
also showed that the plant could be favoured by
freshwater run-off; nevertheless, freshwater contributed by run-off to the areas surrounding the
estuary is absorbed into the soil, by vegetation
or evaporates (Chow et al., 1994). This dynamic
was not considered in the habitat model; therefore, the influence of freshwater may have been
overestimated in the habitat suitability model.
Regarding eradication methods of the plant,
many strategies have been used, such as the introduction of natural enemies, which depends on
factors like size of the plant, density, distribution,
phenotype, and associated microclimate (Bolt &
White, 1992; Charudattan, 2001; Sims-Chilton et
al., 2009, Altfeld & Stiling, 2009). Another common measure is the use of controlled fire (Owens
et al., 2007), being demonstrated that the growth
of B. halimifolia is favoured (Grace, 2002). In
this study, our results suggest that tidal restoration could represent a good passive strategy for
the control of B. halimifolia in tidal estuaries
modified by dikes, which in combination with
mechanical removal of the plant could be used
to avoid germination of new individuals and recovery of older plants.
15293 Limnetica 33(1), pàgina 10, 15/05/2014
CONCLUSIONS
Hydrodynamic regime is an important predictor
of B. halimifolia distribution in Oyambre estuary,
showing that this plant is restricted in its distribution by time inundated and salinity. These results
also showed that changes in hydrodynamic conditions may have a predictable impact on the type
and distribution of vegetation in salt marshes. B.
halimifolia will remain in areas where the tidal
influence is reduced, most likely competing with
vegetation that is better adapted to the new conditions in the estuary.
ACKNOWLEDGMENTS
We especially thank María Recio, Javier Bárcena
and Gorka Bidegain from the IH Cantabria
Institute for their cooperation in the field studies,
numerical modelling of environmental variables and execution of the habitat suitability
model, respectively. We also wish to thank Mercedes Marchese, who read this manuscript and
improved it with her comments.
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